This invention relates to managing signaling states in packet network telephony gateways and more particularly to handling telephony signaling in a way that minimizes state overhead in the gateway while allowing high scalability.
Packet telephony gateways are sometimes optimized to work in concert with common channel signaling protocols. For example, packet gateways operate with Common Channel Signaling System #7 (CCS#7) or with Integrated Services Digital Network (ISDN) PRI. These protocols allow signal processing entirely outside the gateway itself, such as in a dedicated call control system. This has the advantage of avoiding the need for maintaining any call state in the gateways.
For some types of telephony systems, such as those attached directly to analog telephones or to older trunk systems such as Ear and Mouth (E&M) or Channel Associated Signaling (CAS) systems, the gateway is involved in at least low level telemetry aspects of call signaling. In CAS-like scenarios, the gateway also needs to disentangle the channel associated signaling from truly in-band signaling, such as Dual Tone Multi Frequency (DTMF) signaling.
The types of telemetry signals received by the gateway from a telephone include on-hook and off-hook transitions, hook flash, progress tone detection (and possibly analysis) and DTMF detection and possible generation. For trunks connected via in-band signaling to PBXs, analog switches, etc., a gateway needs to hear various progress signals like busy, network congestion, etc. The gateway then needs to process (i.e. "progress tone analysis") and convert the signals into events.
The simplest approach is to back-haul telemetry-like signaling to a signaling or call processing server and treat the signaling in the call processing server identically as in the common channel signaling case. Unfortunately, this fails to scale for a couple of reasons. The number of independent signaling channels can be extremely large, possibly on the same order of magnitude as the number of stations on a local carrier's network. The number of messages generated can also be very large; many more than 2-3 per call as in the common channel signaling case. For example, every digit pressed while dialing a phone number can constitute a message that must be sent to the call process server. The back-haul signaling approach substantially increases network traffic between the gateway and the call processing server and requires additional computing resources to transmit, receive and process the back-hauled signaling.
Thus, a need remains for reducing the overhead required to maintain call states in the gateway while simultaneously providing gateway intelligence that allows the gateway to process telephone events more efficiently.